In a nonstick skillet, cook the onion and garlic in oil, stirring over moderately low heat until onions are pale golden and translucent. Add the wine, water, sugar, salt, and pepper and simmer the sauce for 2 minutes.
Preheat the oven to 450 degrees F.
Season the halibut with salt and pepper and arrange it in a single layer in a baking dish. Top with the onion sauce. Roast in the oven for 15 to 20 minutes or until the fish is just cooked through.
The sauce can be made 1 day in advance, kept covered and refrigerated.
Recipe courtesy of Leslie Glover Pendleton, author of Simply Shrimp, Salmon, and (Fish) Steak